1. Field of the Invention
The present invention relates in general to the field of balancing different attenuation levels in a voice channel of telephone terminal equipment, and more specifically, controlling the voice channel attentuation depending upon the direction of transmission in the telephone terminal equipment, especially for speaker phone type equipment.
2. Description of the Prior Art
In basic telephone terminal equipment, a voice channel may be composed of a series of function units which are considered one sided and may compose the transmission line, a receiver, a telephone and a transmitter. In modern day high quality telephone terminal equipment, such as a speaker phone system, additional functional units may be included, such as amplifiers, microphones, loudspeakers and attenuation elements. The attenuation element is typically controlled by a microprocessor which determines if a transmission or reception channel is currently in operation. German Patent No. 2,714,132 discloses such a circuit arrangement.
During transmission or reception of the telephone terminal equipment, certain functional units will be combined to form a voice channel. Each of the functional units has a defined attenuation level that varies in a known range of tolerance. The sum of all possible deviations of these levels of attenuation for the individual functional units may be highly noticeable in the voice channel. That is, the prescribed range of tolerance for the output level of the voice channel can be downwardly or upwardly transgressed due to the combined effect of the attenuation levels of the functional units. In order to prevent this, the attenuation levels of the individual functional units must be measured during manufacture of the telephone terminal equipment and must be added so that a corresponding, additional attenuation element can be added to the voice channel. This ensures that the output level of the voice channel is within its prescribed tolerance.
During manufacture of the telephone equipment, the above procedure must be performed for each unit and the result is unique to each of these units for every voice channel within the unit.